Temperature regulating circuit for an ink reservoir



Feb 13, 1968 N. E. B'ALL TEMPERATURE REGULATING CIRCUIT FOR AN INK RESERVOIR Filed April 7, 1965 FIG SAMPLING FIG. 3

INVENTOR. NEWTON E. BALL ATTORNEYS United States Patent 3,369,107 TEMPERATURE REGULATING CIRCUIT FOR AN INK RESERVOIR Newton E. Ball, Claremont, Califl, assignor to Nelf Instrument Corporation, Duarte, Calif. Filed Apr. 7, 1965, Ser. No. 446,324 Claims. (Cl. 219499) ABSTRABT OF THE DISCLOSURE A circuit for regulating the temperature of a wax ink reservoir is provided in which an electrical resistance heating element in thermal communication with the ink supply functions also as the temperature sensor. The heater resistor has a large positive temperature coefficient and is connected in series with a regulating transistor and the power supply. A sampling voltage, representing the desired ink temperature, is compared by a second control transistor with the voltage across the heater resistor, the output of the second transistor being connected to control the conductance of the regulating transistor.

lem, designers have resorted to record depositing mechanisms of other types, for example: electrostatic deposition apparatus, systems which write on specially prepared papers of one type or another, systems which use an electric spark or a pressure stylus for leaving a trace, and the like. However, such systems have not proved to be sufiiciently satisfactory that they are considered a desirable replacement for the ink pen recorders. Moreover, conventional ink pen recorders are themselves not completely satisfactory because of the above-mentioned deficiency.

One particular type of recording mechanism which shows promise of overcoming the disadvantages of the above-mentioned systems is the hot wax inking type of mechanism. In this type, a non-aqueous ink, normally solid at ambient or room temperature, is heated and thereby fluidized for deposition on the record medium. A typical ink of this type may be a mixture of chlorinated naphthalene waxes in which a selected organic dye is dissolved. A pen having a tubular opening coupled to an ink reservoir may be employed. In operation, it is desired to heat both the pen body and the reservoir so as to fluidize the wax ink. In a typical configuration, the diameter of the tubular opening of the pen is selected with respect to the surface tension of the fluidized ink so that the ink does not readily flow until the surface tension is broken by contacting the ink at the writing end of the pen with the surface of the recording medium. During writing, the ink flows smoothly from the pen and provides a cushion or lubricating film between the adjacent surfaces of the pen and the recording medium. Moreover, since the recording medium is generally at ambient or room temperature, the wax ink solidifies on the recording medium almost immediately after deposition so that a dry, permanent trace is provided and no Wetting or softening of the surface of the medium occurs as a result of recording.

3,369,l7 Patented Feb. 13., 1968 The reservoir and the pen may be maintained at a suitable elevated operating temperature by means of electrical resistance heating elements or the like. Such elements can be immersed in the reservoir and thermally coupled to the pen. It is desirable to provide a heating ar rangement of the resistive type for a pen and reservoir of the type described which is capable of rapidly fiuidizing the ink when the recorder is initially turned on for operation and of maintaining the ink at a selected elevated temperature while the recorder is in use. Thus, a substantial amount of heat must be initially and rapidly supplied with a lesser amount being more or less continuously supplied at a selected temperature level after the initial quick heat phase is terminated. In addition to supplying heat energy at two different rates to the reservoir and pen heating mechanism, it is desirable to control the amount of energy supplied so that a selected temperature is maintained.

It is therefore a general object of the present invention to provide an improved arrangement for establishing and maintaining a contained quantity of fluidizable material in a fluid state at a selected temperature.

More particularly, it is an object of the present invention to provide an arrangement for controlling the temperature of a fluidizable wax ink.

It is a particular object of the present invention to provide an arrangement for controlling the current supplied to a resistive heating element associated with a hot wax inking mechanism in which the resistance of the resistive heating element is utilized in controlling the current supplied to heating element to maintain the associated hot wax ink supply at a selected temperature.

In brief, arrangements in accordance with the invention include an ink reservoir, a resistive heating element thermally engaged therewith, and an electrical circuit connected to the resistive element and arranged to control the current supply to the heating element in accordance with a selected temperature at which the ink reservoir is to be maintained. The resistive heating element preferably provides a large positive temperature coeflicient so that as the heating element increases in temperature, the resistance thereof also increases substantially. The electrical circuit includes a first transistor connected in series with the resistive heating element to control the level of current supplied thereto, and a second transistor which is coupled to a resistive sampling circuit including the resistive heating element and is connected to control the conductivity of the first transistor. The circuit is in effect a bridge network with the heating element itself acting as the sensing device to revelop a control signal which is applied to the circuit to regulate the level of current which is supplied to the heating element. One particular advantage of the above-described embodiment of the invention is that the heating element can be short-circuited without damage to the circuit which controls the current to the heater. This results from the fact that, should the voltage drop across the heater element reduce to zero or some low level, the second transistor develops a control voltage which turns off the first transistor, thus preventing an excessive current from flowing through the short circuit or through any part of the control circuit.

A better understanding of the invention may be had from a consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a representation of a recording mechanism including the heating element for a hot wax inking system;

FIG. 2 is a circuit diagram showing the resistive heating element and the associated control circuit in accordance with the invention; and

FIG. 3 is a simplified block and schematic diagram presented for the purpose of explaining the operation of the arrangement of FIG. 2.

As shown in FIG. 1, the hot wax inking system may include a drum 12 around which a sheet of paper or other recording medium is transported for writing thereon. A pen 16 is shown attached to a pen body 18 which is in turn connected to a reservoir 20, the entire mechanism being transported transversely of the paper 14 to provide a graphical record thereon as the paper 14 is drawn around the drum 12. A resistive heating element 22 is shown supported on a bracket 23 within the reservoir 20 and also includes a portion 22 which is thermally coupled to the pen body 18 so as to maintain the ink in a fluidized state outside the reservoir and immediately prior to its introduction into the pen 16. The resistive heating element 22 is preferably fabricated of a material presenting a large positive temperature coefiicient and is provided with terminals B and D for connection to the associated current supply circuit.

The temperature regulating circuit in accordance with the invention is shown schematically in FIG. 2 as including the resistive heating element 22 of resistance R connected at points B and D to receive current from a power supply 24 through a first transistor 26 (T A second transistor 27 (T is connected to control the conductivity of the first transistor 26 in accordance with a control signal which is a function of the actual resistance of the heating element 22. A switch 30 is provided to energize the regulating circuit. The first transistor 26 is connected in parallel with a resistor 32 and in series with a resistor 33, the latter of which effectively limits the maximum value of the current supplied to the heater resistance 22. Connected to a point A between the series limiting resistor 33 and the parallel arrangement of the first transistor 26 and the shunt resistor 32 is a sampling resistor 34 in the form of a potentiometer having its movable arm connected directly to the base of the second transistor 27. An adjustment of the potentiometer 34 permits the selection of the desired level of temperature to be maintained in the reservoir of the hot wax inking system shown in FIG. 1. The second transistor 27 has its collector connected to the base of the first transistor 26 with a current path being provided from the common connection between the first and second transistors 26, 27, and one side of the supply 24 via a biasing resistor 28. A rectifier diode 36, polarized as shown, is connected between the point B and a point C at the emitter of the second transistor 27 from which an emitter resistor 29 is connected to a point D which is common to one side of the power source 24, the sampling potentiometer 34 and the heater resistance 22.

For ease of understanding, the same circuit is shown in a somewhat simplified form in the combination block and schematic diagram of FIG. 3. In operation, the heater resistor 22, indicated schematically as a variable resistor because of its large positive temperature coefficient, receives current from the power source 24 (switch 30 closed) via the first transistor 26 and the series resistor 33. The resistor 32, in shunt with the first transistor 26, develops an initial sampling voltage across the sampling resistor 34 so that the control transistor 27 may turn on the transistor 26 when the circuit is first energized. The sampling voltage is developed across the sampling resistor between the points A and D and a selectable portion thereof is picked off by the adjustable arm of the potentiometer and applied to control the transistor 27. At the same time, a portion of the sampling voltage is developed at the point C via the diode 36. Thus, it is the difference of potential between the movable arm of the sampling rheostat 34 and the point C which controls the transistor 27. This in turn provides the drive to turn on the first transistor 26 which is the major element in controlling the current supplied to the heater resistor 22. As soon as the transistor 26 turns on,

crease. This causes a variation in the division of current at point B with the current flowing in the path through the diode 36 and the resistor 29 tending to increase. As the potential of point C rises, the transistor-27 supplies less current through the emitter resistor 29 and thereby develops a reduced base drive to the transistor 26, thus further reducing the current through the transistor 26 which is supplied to the heater element 22. If the temperature of the heater element 22 decreases below a level selected by the setting of the potentiometer 34, R decreases and the control process reverses itself. The circuit thereby acts as a bridge network in which the heater element 22 serves both as the heating element and the sensing device to eilect a control signal for the remainder of the circuit. It may be seen that the heater element 22 can be short-circuited with no damage to the control circuit, since in such an event the voltage at the point A would be reduced to a very low level, relative to the point D, with the result that the transistor 26 would be turned off so that the only current flowing in the path to the heater element 22 would be the extremely small amount permitted by the resistor 32.

There has thus been described a particular temperature regulating circuit in accordance with the invention for heating a hot wax ink reservoir and maintaining the temperature thereof at a selected level. The temperature level is adjustable and, in operation, the circuit simply and effectively maintains the selected temperature. The resistive heating element serves a dual purpose in the circuit, since it develops the heat for the hot wax ink supply and further, by virtue of the change in resistance developed therein in accordance with the temperature of the ink supply, serves as a sensing device to cause the control circuit to maintain a particular level of current in the heater element corresponding to the selected and detected temperature levels. The circuit advantageously protects against damage resulting from a possible short circuit of the hot wax ink supply heater element, since the current control mechanism of the circuit is arranged to be cut ofi in the event of such a short circuit developing.

Although there has been described above a specific arrangement of a temperature regulating circuit in accordance with the invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements falling within the scope of the annexed claims should be considered to be a part of the invention.

I claim:

1. An electrical circuit for controlling the temperature of an ink reservoir comprising a resistive heater element, a first transistor having input and output electrodes, a second transistor having input and output electrodes, means connecting the output electrode of the first transistor in series with the heater element, means connecting the output electrode of the second transistor to the input electrode of the first transistor, means for connecting a power source to said circuit, and means for causing said second transistor to control the conductivity of the first transistor in accordance with both the temperature of the heater element and a selected temperature for the ink reservoir. 7

2. An electrical circuit for controlling the temperature of an ink reservoir comprising a resistive heater element, a first transistor having input and output electrodes, a second transistor having input and output electrodes, means connecting the output electrode of the first -for causing said second transistor to control the conductivity of the first transistor in accordance with both the temperature of the heater element and a selected tempera ture for the ink reservoir, said last-mentioned means comprising a variable resistance coupled to receive at least a portion of the current passed by the first transistor and means for picking oit a predetermined portion of the voltage developed across said variable resistance as a control signal for said second transistor.

3. An electrical circuit for controlling the temperature of an ink reservoir comprising a resistive heater element, a first transistor having input and output electrodes, a second transistor having input and output electrodes, means connecting the output electrode of the first transistor in series with the heater element, means connecting the output electrode of the second transistor to the input electrode of the first transistor, means for connecting a power source to said circuit, and means for causing said second transistor to control the conductivity of the first transistor in accordance with both the temperature of the heater element and a selected temperature for the ink reservoir, said last-mentioned means comprising a current path connected to a common input-output electrode of the second transistor and arranged to vary the potential of said common input-output electrode in accordance with the temperature of the heater element.

4. An electrical circuit for controlling the temperature of an ink reservoir comprising a resistive heater element having a large positive temperature coeificient such that a substantial variation in resistance is developed as said heater element is heated, said element being connected to both heat the ink reservoir and to provide a potential indicative of the temperature of the element, a first transistor having input and output electrodes, means connecting the output electrode of the first transistor in series with the heater element, means connecting the output electrode of the second transistor to the input electrode of the first transistor, means for connecting a power source to said circuit, and means for causing said second transistor to control the conductivity of the first transistor in accordance with both the temperature of the heater element and a selected temperature for the ink reservoir.

5. An electrical circuit in accordance with claim 4 further including a current path connected across said first transistor so as to develop a potential for controlling said second transistor in the event said first transistor is cut oif.

6. An electrical circuit in accordance with claim 4 further including a resistance connected in series with said first transistor and said heater element in order to limit the maximum current through said transistor and said heater element to a safe value.

7. An electrical circuit for controlling the temperature of an ink reservoir comprising a resistive heater element having a large positive temperature coeificient such that a substantial increase in resistance is developed as said heater element heats up, a first transistor having input and output electrodes, a second transistor having input and output electrodes, means connecting the output electrode of the first transistor in series with the heater element, means connecting the output elect-rode of the second transistor to the input electrode of the first transistor, means for connecting a power source to said circuit, and means for causing said second transistor to control the conductivity of the first transistor in accordance with both the temperature of the heater element and a selected temperature for the ink reservoir, said last-mentioned means including a diode connected between the output electrode of the first transistor and a common input-output electrode of the second transistor so as to apply substantially all of the variation in voltage across said heater element to said common input-output electrode.

8. An electrical circuit for controlling the temperature of an ink reservoir comprising a resistive heater element, a first transistor having input and output electrodes, a second transistor having input and output electrodes, means connecting the output electrode of the first transistor in series with the heater element, means connecting the output electrode of the second transistor to the input electrode of the first transistor, means for connecting a power source to said circuit, means for causing said second transistor to control the conductivity of the first transistor in accordance with both the temperature of the heater element and a selected temperature for the ink reservoir, and means for controlling the second transistor to drive the first transistor to cutofi in the event of a short-circuit in the heater element.

9. An electrical circuit for regulating the temperature of an ink reservoir comprising a resistive heater element; a first transistor having input and output electrodes; a current limiting resistance; means connecting said first transistor, said heater element and said limiting resistor all in series; means for connecting the output electrode of the second transistor to the input electrode of the first transistor whereby said second transistor controls the conductivity state of the first transistor; adjustable means coupled to the input electrode of the second transistor for applying a predetermined control potential thereto in accordance with a selected temperature for the ink reservoir; and a diode connected between one side of the heater element and a common input-output electrode of the second transistor for applying a change in potential across said heater element to said common input-output electrode to vary the conductivity of said second transistor in accordance therewith.

10. An electrical circuit in accordance with claim 9 wherein said heater element comprises a material having a large positive temperature coefficient such that a substantial increase in resistance and change in potential are developed as the temperature of said heater element increases.

References Cited UNITED STATES PATENTS 3,215,818 11/1965 Deaton 2l9-499 2,510,041 5/1950 Rudahl 219499 3,028,473 3/1962 Dyer et a1 219504 X 3,067,311 12/1962 Lacy-Hul-bert 210-504 X 3,114,025 12/1963 Blauvelt et a1 219501 X RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner. 

